Interest in crystallization, and in various ways for altering the shapes and structures of crystals, has a long history because an extraordinary range of physical and chemical properties of crystalline solid-state materials is dictated by their crystal form and size. Efforts to modify crystallization processes so as to generate new crystalline forms of substances continue to be of considerable importance for various reasons including, for example, improvement of mass-handling characteristics of particulate materials, production of materials that are stronger or more durable than existing materials, production of materials having improved physical characteristics such as optical clarity, production of materials with long storage period, production of crystalline substance with better flow characteristics, etc.
Conventional ways of altering the shape (i.e., the “habit”) or the crystal lattice (i.e., the “morphology”) of a crystalline material include: (1) using additives (Weissbuch et al., Science 253:637, 1991; Addadi et al., Topics in Stereochem. 16:1, 1986; Addadi et al., Angew. Chem. Int. Ed. Engl. 24:466, 1985; and Addadi et al., Nature 296:21, 1982); (2) changing the crystallization solvent (including crystallization from the gas phase) used to dissolve the crystallization solute; (3) changing supersaturation of the crystallizing solution; (4) and altering the rate of evaporation.
Common salt, apart from being an essential dietary component, is a basic raw material for the manufacture of a wide variety of industrial chemicals viz, sodium carbonate (soda ash), sodium hydroxide (caustic soda), and chlorine. Besides, salt is used in textile, dairy, dyeing, food, fertilizer, paper and pharmaceutical industries. Caking of water-soluble inorganic salt such as common salt is a common storage problem. Caking is believed to occur in such salt because of the formation of solid inter-crystallin bridges that cement crystals together. Evaporation of minute amount of water on the surface of the crystals causes the formation of inter-crystalline bridges and consequently caking over the period of storage time. Understandably, caking reduces free-flow properties of common salt that has got direct negative influence in its use as dietary component and increases storage problem. Besides salt bridge formation, shape of the crystalline particles has got direct influence on the free-flow property of the substance. Larger inter-crystalline surface area contacts, as it is in cubic form, has negative influence on the free-flow properties. Obviously, the inter-crystalline surface contact area is greatly reduced in case of spherical or near spherical crystallites and thereby increasing its free-flow property.
In the prior art (R. Kern, 1953, Compt. Rend., 23b, 830), it is shown that supersaturation has definite effect on the modification of crystal habit of common salt. At a high supersaturation, common salt crystals grow as octahedron ((111) faces) shaped crystals instead of its normal cubic ((100) faces) form. However, these conditions are too extreme to be of any practical use in production of modified crystals of common salts.
In the prior art, Urea is known to modify common salt crystals from cubic to octahedron since 1783 (J. B. L Rome de l'Isle, 1783, Crystallographie, 2, Ed. Paris). However, because of its toxicity, urea cannot be used as habit modifier of common salt for dietary application.
In the prior art (Brit. Patent No. 752, 582, by N. V. Koninklijke Nederlandsche Zoutindustre, 1954), it is claimed that small amount of potassium ferrocyanide (4 ppm by weight) inhibits caking of common salt to a considerable extent. The plausible explanation of the efficiency of potassium ferrocyanide as anti-caking agent is that the habit modifier causes the inter-crystalline caking bridges to become dendritic and therefore friable. Although it finds application where common salts have to be dispersed over a large area such as in de-icing applications in winter, it cannot be used as dietary component because of the possible toxicity of the cyanide compounds.
In the prior art, (L. Phoenix, British Chemical Engineering, Vol-11, 1966, 34), a long list of various habit modifiers and their effectiveness as anti caking agent has been reported. This list includes cyanide salts of various metal ions, cadmium chloride, lead chloride, potassium silico-tungstate, ammonia triacetamide, victamide etc. These agents at low concentrations modify the habit of NaCl crystals from cubic (100) to dendrites of (100) and octahedron. (111) forms. However, none of these additives may be used in NaCl as dietary product due to possible toxicity of the additives and other practical difficulties.
In the prior art, (Scrutton, A. New Sci. Group, Imp. Chem. Ind. PLC, Runcorn, UK. Symposium Papers—Institution of Chemical Engineers, North Western Branch (1985), (3, Cryst. Habit), 3.1–3.13.), it is shown that NaOH can also act as habit modifier of NaCl in an evaporative crystallizer leading to octahedral (111) shaped NaCl crystals. Obviously, both crystallization technique (i.e. evaporative crystallization) and corrosive nature of NaOH habit modifier do not offer any potential to develop a method for generating modified NaCl crystals for dietary application.
In the prior art, (Sasaki, Shigeko; Yokota, Masaaki; Kubota, Noriaki. Iwate Univ., Morioka, Japan. Nippon Kaisui Gakkaishi (2001), 55(5), 340–342.), it is described that the octahedral {111} faces of NaCl crystal appeared in the presence of citric acid when crystallized at an adjusted pH of 2.72. These new faces were never observed at the natural pH (=0.75) of citric acid. Although, citric acid has good health care property, the disadvantage of this method is the requirement of pH adjustment and the fact that only octahedral crystals—which are less spherical in nature compared to dodecahedral crystals of the present invention—are obtained.
In the prior art, (Fenimore, Charles P.; Thrailkill, Arthur. J. Am. Chem. Soc. 1949, 71, 2714), it is described that Glycine, pyridine, betaine, and β-alanine in aq. NaCl solutions modify the crystal habit of growing NaCl; the first causes the formation of rhombic dodecahedra, the others give octahedra. The main drawback of the prior art is that, even though rhombic dodecahedra are obtained: with Glycine, the initial concentration of Glycine required is as high as 10% in saturated brine. Moreover, in the course of the crystallization process, the Glycine concentration continues to increase and a sizable amount of Glycine can co-precipitate along with salt after the saturation limit of Glycine is attained. This would make the process uneconomical and render the salt unacceptable.
The prior art neither points out this weakness nor any solution. Theoretical considerations (A. Julg and B. Deprick, J. Cryst. Growth., 1993, 62, 587; B. Deprick-Cote, J. Langlet, J. Caillet, J. Berges, E. Kassab and R. Constanciel, Theor. Chim. Acta., 1992, 82, 435) suggest that the zwitterionic form of Glycine is getting adsorbed on (110) planes of NaCl and thereby making this face to grow more slowly compared to (100) planes resulting into the formation of rhombic dodecahedron crystals. Glycine is more attractive as habit modifier as it helps develop the (110) faces resulting in rhombic dodecahedron (i.e. nearly spherical) shaped NaCl crystals.
According to Ullmann's Encyclopedia. (2002), Glycine is reported to have a refreshing, sweetish flavor, and occurs abundantly in mussels and prawns. It is considered to be an important flavor component of these products. When used as an additive for vinegar, pickles, and mayonnaise, it attenuates the sour taste and lends a note of sweetness to their aroma. In other prior art [Pillsbury Comp., U.S. Pat. No. 3,510,310, 1970 and C. Colburn, Am. Soft Drink J. 126 (1971)] Glycine is reported to be used to mask the aftertaste of the sweetener saccharin. Glycine is also reported to exhibits a special preservative effect [A. G. Castellani, Appl. Microbiol. 1 (1953) 195. Nisshin Flour Milling, JP 7319945, 1973(G. Ogawa, K. Taguchi); Chem. Abstr. 81 (1974) 76689z. Nippon Kayaku, JP-Kokai 81109580, 1981; Chem Abstr. 95 (1981) 202313b]. The above prior art clearly indicates that not only is Glycine not harmful in any way, it may in fact impart a beneficial effect to certain foods. In the present invention such foods would be those where salt is used and which contains 0.5–1.0% Glycine as additive.